In recent years, in a hygiene material such as a paper diaper, sanitary napkin, an incontinence pad, a water absorbent resin has been widely utilized as a constituent material thereof, from a view point of being excellent in body fluid absorbability. The water absorbent resin is produced via many steps such as polymerization, drying, pulverization, classification, and surface crosslinking (Non-Patent Literature 1). A process for producing a water absorbent resin including such the steps is disclosed in Patent Literatures 1 to 14 etc.
A water absorbent resin is produced by controlling many physical properties (e.g. water absorption capacity, water absorption capacity under load, free swelling rate, liquid permeability, gel stability etc.) as standard, depending on the use purpose thereof (e.g. paper diaper, sanitary napkin etc.). However, in continuous production of a water absorbent resin at a large scale, improvement in productivity, improvement in physical properties and stabilization of physical properties are difficult, even fine variation in physical properties results in deterioration in physical properties of a final product (e.g. paper diaper etc.) and consumer complaint, in some cases. In addition, with increase in a production amount, in a production process including many steps, a small trouble and delay at a part of steps including preparation and carry-in of raw materials cause a great trouble and delay of production, increase in impurities and byproducts, and deterioration in physical properties in a whole production step, in some cases. Then, maintenance of high physical properties and stable continuous operation were a great problem. Further, with increase in a production amount of a water absorbent resin, improvement in productivity thereof was an important problem.
In order to overcome the aforementioned problems, Patent Literature 15 discloses the technique of removing a water absorbent resin having physical properties which is too high or too low, followed by remixing. Patent Literatures 16 and 17 disclose the technique of using a plurality of hoppers at an intermediate step and a polymerization step. And, Patent Literature 17 discloses the technique of performing polymerization in two series and performing a latter half in one series. Further, Patent Literature 19 discloses the technique of adopting at least any one step of a drying step and steps thereafter in two or more series, while a polymerization step is one series, in order to improve or stabilize physical properties. Many techniques of improving or stabilizing physical properties of a water absorbent resin have been proposed by change or impartation of an intermediate step of Patent Literatures 15 to 17 etc., or by use in combination of a plurality of production apparatuses of Patent Literatures 20 to 22 etc., but there was sufficient room for improvement. In addition, in order to improve physical properties, the technique of removing again fine powders, or recycling fine powders after surface crosslinking has been proposed in Patent Literature 23 etc., but removal of fine powders has a problem of reduction in a yield.
And, since as a production amount of a water absorbent resin grows greater, a size of a production apparatus at each step has a limit, this naturally results in that a water absorbent resin is produced in combination in a plurality of production lines. However, in this case, variation in physical properties between respective lines has become a problem. Particularly, in recent years, control of many physical properties, such as not only a water absorption capacity (CRC), but also a water absorption capacity under load (AAP), liquid permeability (SFC, GBP) is required, but in the case of increase in a production amount, or particularly, production in combination of a water absorbent resin with a plurality of production facilities (production lines), control of physical properties is very difficult, many spec out products (off-specification products) are produced, and this is not preferable from a view point of increase in the cost and an environmental burden due to disposal.
And, also in a water absorbent resin including many production steps, a surface crosslinking step is an important step from a view point of improvement and stabilization of physical properties, and reduction in a production trouble, and many improvement techniques have been proposed.
And, various surface crosslinking agents have been proposed, and an oxazoline compound (Patent Literature 24), a vinyl ether compound (Patent Literature 25), an epoxy compound (Patent Literature 26), an oxetane compound (Patent Literature 27), a polyhydric alcohol compound (Patent Literature 28), a polyamidopolyamine-epihalo adduct (Patent Literatures 29 and 30), a hydroxyacrylamide compound (Patent Literature 31), an oxazolidinone compound (Patent Literature 32), a bis or poly-oxazolidinone compound (Patent Literature 33), a 2-oxotetrahydro-1,3-oxazolidine compound (Patent Literature 34), an alkylene carbonate compound (Patent Literature 35), and a polyvalent alkylene carbonate compound (Patent Literature 36) have been proposed. Further, a surface crosslinking method which reduces a volatile alcohol or residual ethylene glycol after surface crosslinking, particularly, after surface crosslinking with an alkylene carbonate compound (Patent Literature 37) has been also proposed.
And, the technique of polymerizing a monomer to crosslink a surface (Patent Literature 38) and the technique of performing radical crosslinking with persulfate (Patent Literatures 39 and 40) have been also known. Further, the technique of using an additive in combination in mixing of a surface crosslinking agent has been also proposed and, as an additive, a water-soluble cation such as an aluminum salt (Patent Literatures 41 and 42), an alkali (Patent Literature 43), an organic acid and an inorganic acid (Patent Literature 44) and the like have been known.
However, although these surface crosslinking methods are a most important production step in a water absorbent resin, in production at continuous operation or a huge scale, there remains room for stabilization and simplification of a production step. Further, in the aforementioned variation in physical properties and generation of spec out, there was a problem of a raw material remaining in a water absorbent resin and a byproduct, for example, a volatile alcohol and residual ethylene glycol described in Patent Literature 37, reduction in them necessitates a complicated step, and the effect thereof was not sufficient.
Main use of a water absorbent resin is a hygiene material such as a paper diaper and a sanitary napkin under the current circumstances, and reduction in a residual raw material (particularly, surface treating agent) and a byproduct is important.
And, particularly when an alkylene carbonate compound is used as a surface crosslinking agent, or a solid non-polymerized organic compound, a representative of which is alkylene carbonate, is used as a surface crosslinking agent, in improvement in productivity of a water absorbent resin, it was necessary to more simplify handleability of a surface crosslinking agent at a production step, and it was necessary to reduce residual ethylene glycol which is generated as a by-product. Particularly, with increase in a production amount of a water absorbent resin, and requirement of higher physical properties, simplification and stabilization of a production step and, further, a problem of a residual byproduct (e.g. volatile alcohol and residual ethylene glycol described in Patent Literature 37) has been viewed as important.
Further, also in a problem of improvement and stabilization of physical properties, a problem of a free swelling rate and liquid permeability is great and, particularly, as a production scale of a water absorbent resin grows greater, maintenance and improvement of a free swelling rate, or realization of both of a free swelling rate and liquid permeability become more difficult, in reality. For maintenance of a free swelling rate and liquid permeability, spec out and reduction in productivity are caused in some cases and, particularly, in the case of improvement in a free swelling rate and liquid permeability, particularly, when an alkylene carbonate compound is used as a surface crosslinking agent, or when a solid non-polymerized organic compound, a representative of which is alkylene carbonate, is used as a surface crosslinking agent, solution thereof was important.